Book Description

Strong electronic nematic fluctuations have been discovered near optimal doping for several families of Fe-based superconductors, raising the question of what role, if any, these fluctuations might play in the superconducting pairing interaction. Underdoped Fe-based superconductors undergo a second order tetragonal-to-orthorhombic structural transition driven by electronic nematic order that accompanies or precedes an antiferromagnetic transition and borders the superconducting dome. In the tetragonal phase Fe-based superconductors are highly sensitive to perturbations of the same symmetry as the nematic order, such as antisymmetric (in this case B2g) strain. Elastoresistivity is described by a high rank tensor (fourth-rank +) as it relates changes in resistivity (second-rank) to strain (second-rank) experienced by a material and is a sensitive probe of broken symmetries. Previously, elastoresistivity measurements of the linear resistivity response to antisymmetric strain have revealed a divergent electronic nematic susceptibility in underdoped materials and has been used to characterize the nematic fluctuations in the tetragonal phase above the zero-field superconducting dome. In this work, I develop two new extensions to elastoresistivity measurements in order to probe the effects of electronic nematic fluctuations in greater detail in a representative family of these materials, the prototypical electron-doped pnictide Ba(Fe(1-x)Cox)2As2. First, I go beyond the first order response via a measurement of the nonlinear elastoresistivity. I find that the strong nematic fluctuations play a large role in the isotropic electronic response of these materials--as evidenced by a diverging nonlinear symmetric elastoresistivity response to antisymmetric strain. Second, I performed elastoresistivity measurements in magnetic fields large enough to suppress superconductivity to investigate the potential that the nematic fluctuations are related to a quantum critical point. I do not observe a magnetic field dependence of the nematic fluctuations and find that they continue to grow with decreasing temperature beneath the zero-field superconducting dome. I performed high magnetic field measurements on samples with a fine distribution of dopings and find that close to the putative quantum critical point, the nematic susceptibility appears to obey power law behavior over almost a decade of variation in composition. This is consistent with basic notions of nematic quantum criticality which, for clean systems, is associated with power law scaling in both doping and temperature. Paradoxically, however, I also find that the temperature dependence of the nematic susceptibility for compositions close to the critical value cannot be described by a single power law.